1. Field of the Invention
This invention relates generally to an optical latch circuit, and more particularly to an optical latch circuit used in an optical-input section of a processor chip in which processing sections and optical-input sections having light-detecting elements are arranged in two-dimensional arrays. Furthermore, this invention relates to a processor unit having optical-input cell arrays in which optical-input cell elements having optical latch circuits are arranged in two-dimensional arrays.
2. Description of the Prior Art
It is well known that currently available architectures of a digital processor have a number of difficulties in further improving high-speed processing performance. Particularly pronounced is the difficulty in transferring electrical signals. These difficulties include complications caused by signal-transfer buses, and limitations in the number of input/output pins for chip packages or carriers for the parallel input of a large amount of information into a single processor chip. In order to overcome these difficulties, an optical interconnecting system has been proposed. The operating speed of a processor is drastically improved if chips in a processor system are directly interconnected optically to enable the high-speed parallel transfer of a large amount of information. One of the critical components in implementing an optical interconnecting system is a light detector that converts low level optical input signals into electrical signals at high speed. An optical interconnecting system which transfers signals between chips, desirably has a light detector with the aforementioned characteristics of converting low level optical input signals into electrical signals at high speed, and is desirably very small. A small parallel light detector does not adversely affect the area for a processing section, a register, a control section, etc. that are the key to a processor chip. In other words, it is desirable to manufacture more than 1,000 extra-small-sized parallel light-detector arrays that exceed the limit of input/output ports for the state-of-the-art processor. In addition, such arrays desirably have sufficient sensitivity and response speed to function at clock rates over the 16-MHz to 32-MHz range normally used in such processor units.
A light detector incorporated in a processor is a solid-state element relying on the internal photoelectric effect. There are two types of photodetectors generally in use, a type relying on the photoconductive effect and a type relying on the photovoltaic effect. Light detectors used to implement small-sized or arrayed devices are generally of a type using the photovoltaic effect. Such small-sized and high-speed light detectors have already been manufactured on a commercial scale through the microfabrication technologies used in the manufacture of semiconductor devices. More recently, various types of opto-electronic integrated circuits (OEICs) in which detecting and processing devices are fabricated on a single substrate have also been widely developed. To meet such varied requirements, the need has arisen to select the optimum material in accordance with the wavelength bands of light received, that is, the absorbed wavelength range of intrinsic semiconductor material, or an electron-excitation wavelength.
A light detector having a high-speed response has been developed as a component of the optical communication systems. The 0.8-1.5 .mu.m wavelength band is widely used for optical communications because this wavelength band involves the lowest optical transmission loss in silica fibers used as a signal transmission medium. In the short-wave band (0.8 .mu.m), silicon-pin photodiodes (Si-pin PDs) and silicon avalanche photodiodes (Si-APDs) are usually used. In the 1.3-1.5 .mu.m band normally used for long-distance communications, Ge-pin PDs, Ge-APDs, InGaAs/GaAs-pin PDs, and InGaAs-APDs are commonly used. Generally, the factors governing the response speed of light detectors are: the speed of dispersion of electrons and holes excited by light in the active layer, and the drift mobility speed at the insulating layer. Since these characteristics are inherent to the materials used to fabricate the devices, high-speed performance of light detectors is ensured by optimizing the thicknesses of the active and insulating layers.
When the electrical characteristics of a light detector for optical communication systems are considered in terms of equivalent circuits, the delay in electron conduction caused by stray capacitance affects response speed greatly. From the viewpoint of signal delay, the equivalent-circuit characteristics of an amplifier installed in-line with the light detector is desirably taken into consideration. In order to optimize the composite resistance and capacitance of the light detector and the amplifier and to minimize variations from circuit to circuit, an OEIC system in which light detectors and amplifiers are fabricated on a single substrate is also being studied.
Another important characteristic of light detectors for optical communication systems is high sensitivity. Because optical signals that are transmitted through long-distance optical fibers are usually very weak, it is desirable to receive them correctly by suppressing noises and increasing the effective relative signal intensity. In order to receive weak optical signals correctly, light detectors having a super-lattice structure, etc. are sometimes used. A detector of this type imposes a super-lattice structure on an electron-hole separating layer to introduce the excited electrons and holes into these layers. This type of light detector prevents the recombination of the holes and electrons and effectively detects weak optical input signals. A light detector of this type, however, requires a sophisticated level of semiconductor fabrication technology.
Light detectors for optical communication systems require high-speed response (greater than one gigabit per second [Gb/s]) and high sensitivity, and they are essentially discrete elements. As a result, light detectors satisfying required characteristics are selected from among multiple discrete elements manufactured by sophisticated fabricating technology.
On the other hand, light detectors, such as the two-dimensional arrays of light-detecting elements used for video cameras, are desirably inexpensive and reliable. Consequently, the materials of light-detecting elements are chiefly Si, which can be fabricated using the most advanced technology. Since an array of light-detecting elements comprises several hundred thousand, or even as many as one million light-detecting elements, its fabrication requires stable and consistent fabricating technology. Light-detecting elements are principally of the Si-pin PD structure, which is a simple light-detecting element structure. The response speed of these light-detecting elements may be lower than that required for light detectors for optical communication systems, mostly up to several megahertz. Light-detecting elements used for video cameras, etc., which usually have human-perceptible light-wavelength bands and a wavelength resolution similar to human visual power, are capable of accurately reproducing light intensity in the perceptible range. The optical signals detected by light-detecting elements are transferred to, or retrieved from, charge-coupled devices (CCDs) or bucket brigade devices (BBDs) in the form of charge packets. In such cases, circuit capacitance, etc. seldom cause failures because of the relatively low-speed transmission.
The light detectors to be used for the aforementioned optical interconnecting systems, however, cannot be of the array structure using CCDs or BBDs to achieve high-speed parallel transmission of signals as in image sensors. In terms of high-speed optical-signal conversion performance, on the other hand, the high-speed capability of more than a Gb/s level detector is not required, but an important consideration here is to make the optical interconnecting light detectors inexpensive and in small-sized arrays.